The present invention relates to the optical scanning of images and in particular photographic images such as still transparencies or cinematographic film to produce electrical signals corresponding to the images, for example for television pictures or video recordings.
Machines to produce such electrical signals from motion picture film, generally referred to as "telecine machines" have been known for many years. References indicate that such a process was known in the 1920's and used by John Logie Baird. Examples of current telecine machines include the "URSA Diamond" (TM) manufactured by Cintel International Ltd. of Ware, Hertfordshire, and the "SPIRIT Datacine" (TM) from Philips of Darmstadt, Germany.
A problem that exists in the use of such machines concerns the visibility in the final television or video image of damage on the original film. Cinematographic film is very fragile, and the emulsion layer which carries the image can be easily damaged in use by particles of dust, handling, friction, abrasion, and in other ways. Damaged areas of the film are immediately noticeable to the eye as light incident on the damaged region is scattered by the uneven surface of the film at that point and the consequent difference in the effective thickness of the film. The granularity of the film may also cause scattering.
Many methods are used to minimise the effect of damage on the video images obtained from film. One widely known technique involves the use of a so-called "wet gate". Such systems are made by Peterson of Wheeling, Ill., USA. According to this technique the film is not scanned in air, as would be the normal case, but in a glass tank filled with a liquid of the same optical refractive index as the film emulsion. Thus, the optical effect of the liquid is to fill the scratch, thereby preventing scattering of the light at damage sites, as the film and liquid form an optically continuous medium of constant refractive index.
These wet gate methods are fraught with difficulty. One problem is that the most widely used liquid of the correct refractive index is perchlorethylene, a known carcinogen. The containment of the liquid in the tank is a second issue that causes difficulty. It is usual for the film to pass through rubber squeegees. These rubber blades act as wipers that rub the fluid off the film. However, the blades inevitably cause damage to the film, such that a slightly damaged film which is passed through the wet gate will generally only be suitable for further use on other wet gate systems as its condition will be worsened by the wet gate process. A further feature that makes the wet gate process undesirable is that the liquids used in wet gates often act as solvents. Thus, the materials that are used inside the tank must be tested to ensure that they will not be damaged by the fluids used. Additionally, the fluids used in wet gates are often flammable, thus making their use highly undesirable.
Other techniques used to minimise the visibility of film damage include methods that illuminate the film with diffuse light. Such known methods include the use of so-called `integrating cylinders`. For example, U.S. Pat. No. 4,868,383 assigned to the Eastman Kodak Company discloses a linear light source for a film scanner including means for generating an intense beam of light and an elongated cylindrical integrated cavity having diffusely reflective walls, and defining an input port through which the intense beam is introduced into the cavity and an output slit parallel to the long axis of the cylindrical integrating cavity to emit a uniform line of light. U.S. Pat. No. 5,241,459, also assigned to the Eastman Kodak Company, discloses an integrating cylinder for use as an illuminator for a film scanner. These methods suffer from difficulties involved in the manufacture of these integrating cylinders, which are very costly to produce, and inevitably have some variation in the evenness of light emitted across the slit.
U.S. Pat. No. 4,937,614, assigned to the Fuji Film Co. Ltd teaches the use of an anisotropic diffusing lens disposed between the transparent original and the light source for diffusing light only in one direction, running along the length of the film, to cope with scratches. Yet again, the manufacture of such anisotropic diffusing elements poses many problems.
It has been realised that in conventional scanning systems with point light illumination, because scattered light is not collected, the scratched and damaged areas are visible in the final image, as less light is collected from these regions. Thus, in a solution to the problem, rather than scanning the film with diffuse light and collecting it at a point detector, the film may be illuminated with light from a point source and the scattered light from the damaged film may be collected. In this way, it has been proposed that the visibility of damage to the film to be scanned, in particular to the emulsion thereof, in the resultant electrical image may be reduced.
PCT patent application WO 83/02869 of Kodak Limited discloses a system for maximising the amount of scattered light which is passed to a detector in the form of a photomultiplier tube. Thus, surrounding a gate in which the film is scanned are mirrors forming a truncated pyramid to capture all of the scattered light and to direct it, together with the directly transmitted light, to a beam splitter. From the beam splitter to each photomultiplier tube there are also provided mirrors in the form of a truncated pyramid. The purpose of the arrangement is to collect as much scattered light as possible, so that the amplitude of the scattered light can nearly equate in value to the average value of the directly transmitted light. However, the scattered light received by a particular photomultiplier tube will not be of the correct colour as it will include scattered light with the wrong colour components. The specification says that this is acceptable since the human eye is much more sensitive to changes in density than to changes in colour in the fine detail of a reproduction. There is thus a possibility that imperfections will show up in a different colour and that the whole colour balance of the image may be affected.
PCT patent application WO 83/02866 of Kodak Limited also discloses a system for maximising the amount of scattered light which is passed to a detector in the form of a photomultiplier tube. This uses a similar system of mirrors to collect scattered light. To increase further the ratio of scattered light detected to direct light detected, this arrangement provides an attenuating filter for the direct light. This suffers from the same problem that imperfections may show up in a different colour and that the colour balance of the image may be affected. The deliberate reduction in the amount of direct light which has passed through the film, using a neutral density attenuating filter, will also reduce the signal to noise ratio.
U.S. Pat. No. 4,481,414, assigned to Eastman Kodak Company, also aims to collect scattered light but uses tapered optical bars, in which there is total internal reflection, rather than mirrors. This system uses trimming filters for the photocells.